The present invention relates to a method for producing polymer profiles having a varying thickness over a width of the polymer profiles and with at least one longitudinal side tapering off to form a thin edge.
When extruding profiles the following problem is encountered: When a uniform extrusion pressure is present, the flow resistance in the thinner profile sections is greater and the flow velocity i s smaller than in the sections of the thicker profile due to the greater closeness of the die walls. As long as the differences in thickness are not to great this effect may be compensated for because the thicker profile sections will pull along the thinner profile sections due to the viscosity of the extrudate while the thicker sections simultaneously experience a certain delay and compression. However, the thinner profile sections are pulled along and accelerated and stretched to such an extent that in a stationary state at a certain distance behind the die the extrudate velocity is equal over the entire width of the profile.
In this context, the term "thin edges" refers to such profile edges in which the tangents in the cross-sectional plane extending through the edge point enclose an angle of less than 50.degree. . Accordingly, the smaller the enclosed angle between the tangents, the thinner is the corresponding edge.
When making these edges thinner and thinner the following problem results: The thinnest profile sections that is the profile sections in immediate proximity to the edge, may not be pulled along to a sufficient extent by the thicker profile sections (based on the viscosity of the extrudate) so that they are partially retained and form irregular seams. When this effect is observed, the corresponding edges must be made thicker or the small chamfer (edge trimming) must be increased However, for constructive reasons, the chamfer for thin edges should also be very small. Accordingly in the extrusion of most profiles, for example, side walls of vehicle tires made of rubber mixtures, the process is always carried out at the borderline of forming irregular seams.
When operating within these slim limits, a further problem is that the thin edges, which are stressed to a point close to their rupture limit, and the compressed thicker profile sections try to compensate the different stress levels by exiting from the die (the extrusion die is viewed from the side) in a bent shape whereby the thin edges follow a smaller radius and the thicker profile sections follow a greater radius. The resulting bent section extends in the plane which is perpendicular to the main extension of the profile cross-section. If the profile to be extruded has a curved cross-section, this curvature will be increased by the extrusion step; this effect is usually not detrimental to the product. However, the aforementioned undesired bending may only be reproduced within certain limits and is thus detrimental to the manufacturing process. For example, the bending of the profile impairs the extrusion of cover layers for vehicle tire side walls having two thin edges and of tread portions for tires. The curving effect also prevents the extrusion of wiper blades which are provided with a thin edge. Instead, such wiper blades must be manufactured by a molding process.
In the recent U.S. Pat. No. 4,981,637, a method has been disclosed for forming a wiper blade in a continuous extrusion process. The die opening is shaped to produce a pair of wiper blades joined edge-to-edge. The pair of wiper blades extends in the same plane. The undesired curving effect, however, may not be prevented with this extrusion method.
It is therefore an object of the present invention to prevent or suppress the bending effect for extrudates having thin edges.